Kt-76c intermittent mode C

[Jcmtl]

My kt-76c is dropping mode c intermittently.  The transponder has a digital screen which shows alt (when working) so it’s easy to know when mode c has been dropped.  The altitude is replaced with dashed lines.  It then comes back back on and drops again. 
 

More info:

-when on ground using battery power, it stays on.

-when alt is showing, it’s the correct altitude.

-when engine is on, it drops intermittently.  
 

i don’t think it’s the antenna.  Any avionics guys out there have any ideas.  My shop has looked at this, thought it was the encoder fuse, replaced it, but problem persists.  

[N201MKTurbo]

I would check your encoder wiring.

[LANCECASPER]

15 hours ago, Jcmtl said:

My kt-76c is dropping mode c intermittently.  The transponder has a digital screen which shows alt (when working) so it’s easy to know when mode c has been dropped.  The altitude is replaced with dashed lines.  It then comes back back on and drops again. 
 

More info:

-when on ground using battery power, it stays on.

-when alt is showing, it’s the correct altitude.

-when engine is on, it drops intermittently.  
 

i don’t think it’s the antenna.  Any avionics guys out there have any ideas.  My shop has looked at this, thought it was the encoder fuse, replaced it, but problem persists.  

Do you have a blind encoder or an encoding altimeter?

[Jcmtl]

1 hour ago, LANCECASPER said:

Do you have a blind encoder or an encoding altimeter?

blind encoder

[LANCECASPER]

51 minutes ago, Jcmtl said:

blind encoder

Since the KT76C has a cavity tube it could be that starting to fail, it could be the encoder itself or the wiring between the two. Might be time to start thinking about an install of the best transponder made today . .  the Garmin GTX345 with the small Garmin encoder attached to the back.  ADS-B out of course, plus IN which is compatible with pretty much any display installed or portable, back up AHRS, bluetooth, etc, etc.

[flyboy0681]

22 hours ago, LANCECASPER said:

Since the KT76C has a cavity tube it could be that starting to fail, it could be the encoder itself or the wiring between the two. Might be time to start thinking about an install of the best transponder made today . .  the Garmin GTX345 with the small Garmin encoder attached to the back.  ADS-B out of course, plus IN which is compatible with pretty much any display installed or portable, back up AHRS, bluetooth, etc, etc.

I agree that it's probably the tube. When my original KT76A died several years ago I wanted to take the least expensive way out and had my shop install a yellow tagged KT76C. The unit worked for a couple of years at which time I procured another KT76C for a very little, and it too died within a year. I finally gave in and purchased a new Trig TT31, which is a slide-in replacement. It may be time for you to also bite the bullet.

[N201MKTurbo]

But a tube would cause a mode A failure too. It couldn’t cause just a mode C failure, and tubes aren’t usually intermittent, their cathodes lose their emissivity and their output power drops until it isn’t enough to work. 

[carusoam]

Sounds like a loose wire, affected by vibration…

We can ask @Alan Fox…. He may have replacement parts for anything that has failed….

Best regards,

-a-

[Jcmtl]

On 6/1/2022 at 12:07 AM, carusoam said:

Sounds like a loose wire, affected by vibration…

We can ask @Alan Fox…. He may have replacement parts for anything that has failed….

Best regards,

-a-

It was a loose wire.  It’s fixed. 

[carusoam]

Great follow-up Jc!

Go MS!

Best regards,

-a-